Hypomelanism is a recessive genetic mutation in some animals causing malfunctioning melanin (pigment) production or problems with its transport to pigmented cells. What does this mean in practical terms? Melanin is the thing that browns us when we tan. It also makes the patterned markings on Bearded Dragons as well as other things on them dark. A Dragon that has a genetic mutation that either inhibits the production of melanin or the effective transport of melanin to the parts of the body that it would otherwise darken is said to be hypomelanistic. Where this is the case, the animal will generally appear lighter all round.
There are some key factors in determining if a Dragon is definitely hypo. A Dragon can be lighter and indeed almost white, without being hypo. As this is a genetic trait rather than polymorphism, which is the term describing non genetic colour variation in animals, to be a true hypo key expressions of the trait must be present through generations. The easiest trait to identify is that true hypos have clear nails on all their toes. Normal Bearded Dragons have black nails. If you have a Dragon that has clear nails, then it is a hypo, the traits are genetic and they can be passed on to future generations through selective breeding. Hypomelanism is not the same as Albinism where no melanin is produced at all, which, in Bearded Dragons, is not a terribly desirable trait as it is something that has the potential to severely shorten their life and generally does. But, that is another discussion. You can find more information on this and other Bearded Dragon genetic traits on this page - LINKY.